Pressure-cast-in-situ pile with spray-expanded frustum (PPSF) is a new type of variable cross-section pile. To clarify uplift bearing mechanism of the PPSF, a series of numerical simulations of the PPSF are conducted. Criteria for determining the shear failure modes of the rock and soil surrounding the PPSF are established based on the numerical simulation results, and a calculation method for predicting the ultimate uplift bearing capacity of the PPSF considering both general shear failure and local shear failure is proposed. The present study shows that the oblique extrusion effect of the upper frustum can mobilize the cooperative bearing of additional soil around the PPSF, significantly improving the ultimate uplift bearing capacity of the PPSF. The internal friction angle of the soil surrounding the PPSF has a significant impact on the uplift bearing characteristics of the PPSF, while the effect of cohesive force on the uplift bearing capacity of the PPSF can be ignored. With constant cohesion, the ultimate bearing capacity of the upper frustum increases significantly with the internal friction angle. The ultimate end resistance of the upper expanded frustum increases with both the length of the pile above the expanded body H and the expanded body-to-pile diameter ratio (D/d ratio).